Before I took classes in soil mechanics, I wasn't afraid of liquefaction. Before I took a materials science class, I wasn't afraid of creep - there was no such thing as deformation under constant stress; if it was holding now, it would hold forever. I knew the concept of fatigue1, but I didn't lose sleep over it before I understood the effect of stress concentrations and stress corrosion.
I wonder what else I'll learn to fear during my engineering career.
1Don't let anybody tell you that bending a paper clip back and forth until it breaks is an example of metal fatigue. When the paper clip is bent that way, it deforms plastically; by definition, fatigue occurs within the elastic region.